Conventional disk drives employ a servo system which controls the radial position of an actuator arm relative to a rotating recording disk. The actuator arm supports a read/write head or transducer in cantilevered fashion above the disk. Ideally, the read/write head is maintained over the center of a selected track in the disk surface. The read/write head senses position information from the track, which is then used to develop a position error signal. The error signal is fed back through a compensator into a drive motor for the actuator arm to move the read/write head in a direction to reduce the error.
At one time, disk drives were employed primarily within immobile computers, where the disk drives sat motionless within the computer on a desktop or table. There was little likelihood that these drives would experience an immense shock from being dropped or severely jolted. Most external disturbances were in the form of vibrations which could, if sufficient in strength, cause the disk to move relative to the read/write head, resulting in an off track error.
Today, an increasing percentage of disk drives are being used in portable computers, such as laptops, notebooks, and palmtops, and other portable devices. A primary problem associated with contemporary designs of such disk drives concerns shock-induced damage created by the impact of a falling portable device colliding with a surface. For example, when a palmtop computer containing a small disk drive (such as a 1.3 inch disk drive) falls from a desk or user's hand onto a hard floor, the shock pulse magnitude generated at impact can be hundreds or thousands of g's.
The external shock yields two undesirable outcomes: physical damage and track misregistration. With respect to the first, the shock pulse may cause significant physical damage to the disk drive. A pulse of sufficient magnitude and orientation can overcome the air gap bearing which separates the slider from the rotating disk, thereby damaging the magnetic media film, the disk substrate, or the slider.
With respect to the second, the external disturbance imposes linear and radial accelerations that apply a moment to the actuator arm. This moment may exceed the tracking servo systems ability to maintain the read/write head within the allocated tracking error range required for acceptable data integrity. This problem is exacerbated by increased track density which reduces the acceptable tracking error range, and by the trend towards smaller form factor disk drives which are subjected to more rigorous operating environments. If a shock occurs during the data writing process, the disk drive is in jeopardy of miswriting the data off track, or worse, over previously written data on adjacent tracks.
To compensate for minor external disturbances, some prior art disk drives use an accelerometer with the servo system. The accelerometer senses acceleration caused by vibrations and the like. There are two primary techniques for using the accelerometer output to accommodate disturbances. The first is to actively and controllably compensate for the effects of the vibrations. The actuator is controlled to shift the read/write head in the direction of displacement effective to cancel the vibrations and eliminate the off track error.
The second technique is a write protect function designed to reduce the misregistration errors caused by external disturbances. The write protect function is triggered by large net accelerations output from the accelerometer. When acceleration exceeds a prescribed level (typically 2.5-10 g's for translational sensors), writing is interrupted before any data is written off track, thereby preventing data loss or misregistration. However, the write protect function is not very effective for alleviating the effects of an impact shock resulting from a free falling disk drive. The nominal acceleration of a falling disk drive is only 1 g, which is insufficient to initiate the write protect function. The read/write head remains over the data tracks. Upon impact, the acceleration easily exceeds the 2.5-10 g threshold, thereby enabling the write protect function, but the read/write head is still positioned over the disk which can cause significant physical damage to the disk drive.
The present invention concerns disk drives and associated methods for detecting shock-inducing events, such as free fall conditions, and taking protective action to minimize the damage and errors induced by such shock.